Diesel engine after-treatment systems that use selective catalytic reduction (SCR) are generally rather large, typically using long exhaust pipes in order to completely evaporate injected aqueous urea solution and sufficiently mix the injected solution with the exhaust gas. For that reason, packaging such a system poses a challenge to vehicle engineers and designers, and adds weight and cost to a vehicle.
Furthermore, heat losses through the system walls to the outside environment may lead to incomplete evaporation of the injected urea solution, incomplete conversion of urea into ammonia, and the formation of solid deposits in the exhaust system, all of which are typically to be avoided because they have adverse effects on the life and performance of the after-treatment system.
Because it is conventional design practice to place a urea injector downstream of a particulate filter, the injector is at times (e.g., during active regeneration of the DPF, either controlled or uncontrolled) exposed to exhaust gas temperatures that may exceed the allowable maximum temperature for which the injector is designed, and consequently the injector may be damaged over time because of such repeated exposure.
One way to reduce the overall size of an after-treatment system is by improving the efficiency with which injected urea solution is evaporated. Greatly increasing the surface area of solution entraining with exhaust gas flow, such as by using compressed air to reduce the size of spray droplets while greatly multiplying their number, increases evaporation efficiency. Although that is one way to reduce the size of the after-treatment system, a source of compressed air may not be present as existing equipment in some vehicles, and if an auxiliary air compressor is added to provide the compressed air source, it too will be packaged in the vehicle and that of course would add to the cost of the after-treatment system as well as consume energy from the engine to compress air.
An alternative to using compressed air is to evaporate urea outside of the exhaust line and inject ammonia gas, such as described in SAE (Society of Automotive Engineers) paper 2006-01-0642, but that too adds to the after-treatment system cost and involves somewhat bulky equipment that will also be packaged in the vehicle.
While insulation (dual-wall exhaust pipe, for example) can reduce heat losses that could otherwise impair evaporation efficiency, insulation doesn't provide a total solution, and it too adds to the after-treatment system cost.